Façade works have begun at the $3.2bn One World Trade Center in New York, US.
Stainless steel and 12,000 glass panels are being installed on the façade of the 164.9m² skyscraper.
The façade panels will form eight triangles of glass and steel, which grow alternately wider and narrower as they approach the top of the building.
Designed by Skidmore, Owings & Merrill, the tower's exterior refracts light depending on the weather and position.
A porous façade made of prismatic glass and an aluminum screen will cover the first 20-floors of the building.
The building is touted to become the tallest in the city upon completion in 2013, according to dnainfo.com.